1. Technical Field
The present invention relates to studs for use in high temperature applications such as boilers and furnaces. The studs are made of an iron aluminum alloy. They are welded to surfaces of the boilers or furnaces and provide additional heat transfer area or anchor refractory material to the surfaces.
2. Description of the Prior Art
Furnaces and boilers having studded surfaces may be fueled by a variety of fuels such as coal, petroleum and natural gas. Studs are also used in such furnaces or boilers as waste heat boilers, solid waste incinerators, and paper mill tar boilers.
Many components of these furnaces or boilers, such as heat exchanger tubes, have surfaces which are exposed to high temperatures and corrosive conditions. Resistance to the high temperatures and corrosive conditions is of great importance.
The studs are small metal projections which are welded onto the surfaces in a closely spaced pattern. The studs can have a variety of configurations. On a heat exchange surface, the studs provide additional surface area for heat transfer. They can also protect the surface from direct exposure to combustion heat and by-products which reduce the life of the furnace component. They can also protect the surface from abrasion caused by the motion of slag, carbon particles, or particulates present in the combustion gases. On a refractory covered surface, the studs function primarily as anchors for the refractory material.
The studs used presently in furnace applications have a mild steel, stainless steel or nickel based composition. Mild steel studs have been widely used for low temperature applications. For high temperatures and corrosive conditions, stainless steel alloys and nickel based alloys are used. The most commonly used stainless steel grade of ferritic alloy is SS430. Ferritic stainless steels have increased heat and scale resistance due to their chromium content of 11 to 18 weight percent. Another class of stainless steels which is used are the austenitic alloys such as 304, 305, 309, 310, 316, 316L, 321 and 348. These stainless steels typically have 17 to 26 weight percent chromium and 8 to 22 weight percent nickel.
For ultimate temperature performance, studs made from nickel based superalloys like INCONEL are used. These alloys can have nickel in the range of 40 to 70 weight percent and chromium in the range of 15 to 48 weight percent. Costly alloying elements like nickel and chromium in both the stainless and nickel based superalloys increase the cost of the raw material.
The studs are usually welded to the surface of a furnace component by either stud arc welding or resistance welding. In both of these procedures, the studs are one of the electrodes in the welding step.
The strength of the resulting weld is a good indicator of the weld quality which is an important factor in determining stud life in service. Even though some studs presently used may be considered to be essentially sacrificial in nature, the replacement of studs is expensive and time-consuming and can require substantial down-time for the installation involved. It is therefore desirable to have as high a weld strength as possible, in order to achieve the longest stud life.
To achieve optimum weld strength, it is necessary to provide a flux which during the weld cycle will function as a getter to absorb gases generated by the weld process. The flux inhibits the formation of voids in the finished welds.
In the stud welding process, the most frequently used flux is aluminum. Aluminum in the molten state is an excellent absorber of gases and at the same time has melting and freezing points which permit its use in the welding of iron and nickel based materials.
U.S. Pat. No. 2,883,215 discloses a steel stud provided with an aluminum flux. The stud is formed with a frustoconical end. The end has a flat surface, a hole in the flat surface, and an aluminum flux ball fitted into the hole.
The stud may also be aluminum fluxed by coating aluminum on the surface of the stud, or by positioning powder aluminum in a cap on the end of the stud.